Researchers from the Zoological Society of London (ZSL) say the trick is helping them study the disease-causing micro-organisms carried by live whales.

It is an importance advance given that much of what we know about the cetaceans comes from dead animals, not free-ranging ones.

ZSL's Dr Karina Acevedo-Whitehouse told the BBC: "We don't know much about them because they are so big and they are in the water all the time, and that makes it really difficult to obtain biological samples that are relevant to determining health in these populations; unless they've already stranded or unless they are in captivity, which are hardly representative of a normal population."

Karina Acevedo-Whitehouse scans the water for whales

The chopper breathalyser is featured in the BBC's new Oceans series, which records a series of underwater scientific expeditions aimed at building up a global picture of the state of our seas.

The ZSL work is being undertaken in the Sea of Cortez (also known as the Gulf of California), where sperm whales (Physeter macrocephalus) gather in large numbers.

The mammals have been boosted by the presence of abundant prey - humboldt squid (Dosidicus gigas), which themselves seem to be thriving in what are generally over-fished waters.

The whale numbers give plenty of opportunities for the researchers to get close to the animals - but often not close enough to get a good breath sample.

In the Oceans programme, presenter Lucy Blue is seen hanging out over the end of a small boat with Petri dishes attached to a long pole. But every time she gets near a whale, the creature dives. XXX

The one-metre-long remote-controlled chopper allows Dr Acevedo-Whitehouse's team to take a sample without the whales being disturbed. The "pilot" controls the aircraft at a safe distance, directing the machine to hover over the water where whales are surfacing.

When a whale blows, the dishes catch the spray thrown up by the animal. The dishes are then flown back to the control boat where they are sealed and taken to be analysed in a laboratory to identify particular pathogens - bacteria, viruses, fungi.

"We're on a moving boat and we don't have too much space to manoeuvre, to let the helicopter land, so it can be quite dangerous," Dr Acevedo-Whitehouse told the BBC World Service's Science In Action programme.

"Also, it's quite challenging to be able to calculate at a distance where the whale is and where the blow is. We've even attached a tiny video camera to the helicopter to see when the whale is actually blowing and where we could pass the helicopter through it to collect the sample."

The helicopter has to be handled with care on a small boat

Using both the chopper and the pole method, the team has managed to sample some 60 whales in the Sea of Cortez (and a further 40, of different species, around Gibraltar).

The ZSL research is a collaborative study with Dr Diane Gendron from CICIMAR, a research institution belonging to the National Polytechnic University of Mexico.

"[Dr Gendron] has been working with the whales for more than 20 years," said Dr Acevedo-Whitehouse.

"She can photo-ID the animals; we know what animal it is. We will be able to tell if a whale is having the same pathogens year after year. It will let us monitor the health of the populations."

The BBC Oceans series starts its run on BBC Two at 2000 GMT, Wednesday 12 November. You can also hear an interview with Dr Karina Acevedo-Whitehouse on Science In Action from Friday

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